Fix issue with IAM role credentials and default constructor for DynamoDBConfigurationSource

The DynamoDbConfigurationSource's default constructor will getCredentials from DefaultAWSCredentialsProviderChain, which is fine if the credentials are in environment variables or system properties. Unfortunately with IAM roles, the credentials will expire, and since the credentials rather than the credentials provider was used to create the AmazonDynamoDBClient, eventually there will be an issue.

This pull request instead uses the the credentials provider instead of just the credentials. In addition, new constructors have been added DynamoDbConfigurationSource to mirror the constructors of AmazonDynamoDBClient, so end users can specify AWSCredentialsProviders and ClientConfigurations if they so desire.